The present invention relates to electrical amplifiers, and more particularly to audio power amplifiers.
A Class A audio power amplifier includes a differential amplifier driving a transconductance stage. Class A amplifiers are typically biased, meaning that the transistors conduct (i.e. remain active) during the entire input cycle. To bias the transistors, voltages or currents are applied to the transistors in the amplifier circuit to establish the threshold operating point for each transistor. The current that results in the amplifier circuit from biasing, measured before any signal current is applied, is defined as the quiescent or idling current.
In any amplifier system, three properties determine the purposes for which the amplifier is suited: gain, linearity and efficiency. Gain is the amplification factor of the circuit. The output parameters (voltage or current) are a function of the input parameters multiplied by the gain of the amplifier. Linearity is the extent to which a linear relationship exists between the input and output parameters. The output parameters should exactly correspond to the input parameters multiplied by the gain of the circuit. Any nonlinearities introduced by the circuit will result in a distorted output signal. Finally, the efficiency of the amplifier is the ratio of the output or load power to the input or supply power. Efficiency measures how much of the input power is consumed by the circuit.
To enhance the output, one or more Class B current amplifying buffer stages can be interposed between the transconductance stage of a Class A amplifier and the output devices. A traditional Class B amplifier conducts for half of each cycle which allows for greater efficiency than the Class A amplifier. A typical Class B stage consists of two parallel transistors, one of these transistors is an N-channel type which reproduces the positive half of the cycle, while the other is a P-channel type that reproduces the negative half of the cycle. The two half-cycles are then recombined at the output to produce an amplified version of the original input wave form. However, since Class B amplifiers amplify each half of the input signal by separate transistors, any difference between the transistors result in nonlinearities that contribute to the distortion of the amplified output. Class B amplifiers are also plagued by crossover distortion that occurs as the input signal switches from positive to negative. At crossover, both transistors are off and there is a slight delay until the proper transistor begins to conduct the input signal. Crossover distortion in audio power amplifiers gives rise to unpleasant sounds.
To reduce crossover distortion, the biasing system of a Class B amplifier is adjusted so that the transistors conduct at a point somewhere between the traditional Class A and Class B amplifiers. This hybrid design is known as a Class AB amplifier. In a Class AB amplifier the two parallel transistors of the Class B amplifier are biased with a small, non-zero current. This biasing current eliminates the problem of crossover distortion because both transistors conduct when the input signal switches from positive to negative. Thus, the delay is eliminated as the transistors begin to conduct and crossover distortion is eliminated.
As would be expected, the linearity and efficiency characteristics of Class AB amplifiers fall between those of a true Class A or Class B amplifier. Designers have been reluctant to forego the linearity of Class A amplifiers in favor of the more efficient, but less linear, Class AB systems. Therefore, most audio power amplifiers include the Classic Class A amplifier topology for the driver stage.
Further expanding on the foregoing discussion, FIG. 1 illustrates a specific prior art classic Class A amplifier system. The transconductance stage includes transistors Q1 and Q2 biased by an appropriate biasing system, diodes D1 and D2, and Resistors R1, R2, R3, and R4. The transconductance stage acts as an amplifier in which the output current is a linear function of the input voltage. The complimentary bipolar output transistors Q1 and Q2 are arranged in a push/pull configuration with the output transistors Q1 and Q, biased by an appropriate biasing system. The output devices are connected to the collectors of the output transistors Q1 and Q2 with current provided to the output devices in accordance with the current flowing through the emitter junctions of the output transistors Q1 and Q2 as controlled by the signal from the differential amplifier stage.
The differential amplifier stage consists of transistors Q3 and Q4, resistors R5 and R6, and a current mirror M. The signal to be amplified is received at the base of either Q3 or Q4 which then induces a signal current in the corresponding output path from the differential amplifier to the transconductance stage. The two paths from the differential amplifier to the transconductance devices are developed and processed independently and linearly. This arrangement results in a Class A amplifier with very good linearity, but with low efficiency. Class A amplifiers are typically biased such that the transistors conduct during the entire input cycle which results in the low efficiency because most of the power is consumed by the circuit. Additionally, these Class A audio amplifiers are typically limited to a 2:1 peak-to-quiescent current amplification--that is, the output current is limited to a factor of two times the idling current present in the circuit. This output current is inadequate to drive the desired output devices.
Minimizing the number of stages in the signal path usually results in significant improvements in the stability of the amplifier and the resulting quality of the output. However, as shown in FIG. 2, the Class A audio power amplifier design of FIG. 1 can be modified by the addition of Q5, Q6, and Q7 in a common-base or cascode configuration. Q5 and Q7 are N-channel type bipolar junction transistors where the bases are established at +/-1/2 of the supply voltage, respectfullyl. Q6 is a P-channel tppe bipolar junction transistor with its base set to +1/2 of the supply voltage. As is known in the art, the cascode configuration results in greater stability due to the increased frequency response resulting from a reduction of Miller-effect capacitances.
A significant gain in the short-loop design art has been possible due to the use of power metal-oxide semiconductor field effect transistor (MOSFET) output stages. These stages can be driven directly from an appropriate transconductance stage. To date, these bufferless designs have been effected by increasing the power level of the Class A transconductance stage; however, this process quickly becomes unwieldy due to the resulting heat and related effects that occur in the individual components. Even with these difficulties associated with the traditional audio power amplifiers, and as noted above, designers have been reluctant to forego the high quality sound reproduction capability of the highly linear Class A transconductance systems in favor of a more efficient, Class AB system.